Liquid crystal displays are widely used in watches and clocks, photographic cameras, various instruments, computers, flat television sets, projection screens, and numerous information devices.
Electro-optical modes employed in LCDs include, in particular, the twisted nematic (TN), super twisted nematic (STN), optically compensated bend (OCB), and electrically controlled birefringence (ECB) modes, as well as some others and with their various modifications. All these modes use an electric field, which is substantially perpendicular to the substrate and, hence, to the liquid crystal (LC) layer. Besides these modes, there are several electro-optical modes employing an electric field substantially parallel to the substrate and, hence, to the liquid crystal layer, for example, the in-plane switching.
The in-plane switching (IPS) and vertically aligned (VA) modes are the most widely used in LCDs for large scale modern desktop monitors and TV sets, and are envisaged for use in future displays for multimedia applications.
A TN (twisted nematic) mode LCD is a common type of conventional LCD using liquid crystal molecules that have positive dielectric anisotropy and are horizontally aligned in a twisted state between two substrates. However the TN LCDs cannot display an absolute black state because of hardly compensated light leakage. On the other hand, the IPS LCD can display an almost complete black state in an OFF-state because the liquid crystal molecules are aligned almost horizontally and uniformly by the surfaces of the substrates so that the light polarized linearly along the LC optical axis undergoes no change in polarization state when passes through the liquid crystal layer. The VA mode LCD is also characterized by uniform distribution of LC molecules in the OFF-state. However, for a high-quality optical compensation of VA LCD one needs using at least two different types of the retardation films. Thus the light leakage remains a problem to be solved.
In connection with polarizing plates, compensation panel, retardation layers described in the present application, the following definitions of terms are used throughout the text.
The term optical axis refers to a direction in which the different linearly polarized components of propagating light have the same phase velocity and do not exhibit mutual retardation.
Any optically anisotropic medium is characterized by its second-rank dielectric permittivity tensor. A dielectric permittivity of any medium is determined by polarizability of particles forming this medium. If medium comprises supramolecules then dielectric permittivity of the medium is determined by orientation and polarizability of these supramolecules.
The classification of compensation panels is tightly connected to orientations of the principal axes of a particular permittivity tensor with respect to the natural coordinate frame of the compensation panel. The natural xyz coordinate frame of the panel is chosen so that the z-axis is parallel to the normal direction and the xy plane coincides with the panel surface.
FIG. 1 (prior art) demonstrates a general case when the principal axes (A, B, C) of the permittivity tensor are arbitrarily oriented relative to the xyz frame. Orientations of the principal axes can be characterized using three Euler's angles (θ, φ, ψ) which, together with the principal permittivity tensor components (∈A, ∈B, ∈C), uniquely define different types of optical compensators. The case when all the principal components of the permittivity tensor have different values corresponds to a biaxial compensator, whereby the panel has two optical axes. For instance, in case of ∈A<∈B<∈C, these optical axes are in the plane of C and A axes on both sides from the C axis. In the uniaxial limit, when ∈A=∈B, a degenerated case takes place when the two axes coincide and the C axis is a single optical axis.
In another example two principal axes A and B of the dielectric tensor lie in the panel plane, while the C-axis is normal to it. The x, y and z-axes of the laboratory frame can be chosen coinciding with A, B and C axes respectively. If, for instance, the lowest and highest magnitudes of three principal values ∈A, ∈B, and ∈C of the dielectric permittivity tensor correspond to the A and B axes respectively, then ∈A<∈C<∈B, and two optical axes belong to the AB plane. For this reason such retardation layer is named “AB” or “BA” type panel (FIG. 2, prior art). The negative AB panel, when ∈A−∈B<0, is equivalent to positive BA panel or plate (replacing the order of the naming letters changes the sign of the dielectric permittivity difference: ∈B−∈A>0). Another fundamentally different case is when two optical axes belong to the plane orthogonal to the panel surface. This case takes place if the lowest or highest magnitude of one of the principal permittivity corresponds to the C-axis. For instance, in case of ∈C<∈B<∈A the retardation layer is named negative CA or positive AC panel.
The zenith angle θ between the C axis and the z axis is most important in the definitions of various compensation types. There are several important types of uniaxial retardation layers, which are most frequently used in practice for compensation of LCD.
If a panel is defined by Euler angle θ=π/2 and ∈A=∈B≠∈C then it is called “A-panel”. In this case the principal C-axis lies in the panel plane (xy-plane), while A-axis is normal to the plane surface (due to the uniaxial degeneration the orthogonal orientations of A and B-axes can be chosen arbitrary in the plane that is normal to the xy-surface). In case of ∈A=∈B<∈C the panel is named “positive A-panel” (FIG. 3(a), prior art). Contrary, if ∈A=∈B>∈C the panel is named “negative A-panel” (FIG. 3(b), prior art).
A C-panel is defined by the Euler angle θ=0 and ∈A=∈B≠∈C. In this case, the principal C axis (extraordinary axis) is normal to the panel surface (xy plane). In case of ∈A=∈B<∈C, the panel is named “positive C-panel”. On the contrary, if ∈A=∈B>∈C, the panel is named “negative C-panel”. FIG. 4 (Prior art) shows the orientation of the principal axes of a particular permittivity tensor with respect to the natural coordinate frame of the positive (a) and negative (b) C-panel. The axes OA and OB located in a xy plane are equivalent.
Generally when the permittivity tensor components (∈A, ∈B, and ∈C) are complex values, the principal permittivity tensor components (∈A, ∈B, and ∈C), the refractive indices (na, nb, and nc), and the absorption coefficients (ka, kb, and kc) meet the following conditions: na=Re[(∈A)1/2], nb=Re[(∈B)1/2], nc=Re[(∈C)1/2], ka=(4π/λ)Im[(∈A)1/2], kb=(4π/λ)Im[(∈B)1/2], kc=(4π/λ)Im[(∈C)1/2], where λ is a free space wavelength.
The optical characteristics of LCD devices can be improved by application of one or more layers having optical birefringence. In the conventional commercial displays the retardation layers (or retardation films) are used in order to solve the problems of low contrast and light leakage. The typical retardation film consists of at least one homogeneous layer of uni- or biaxial birefringent material, and is disposed between a polarizer and a liquid crystal cell. The retardation film for compensation of contrast ratio at oblique viewing angles comprises a negative C-type panel for compensating an in-plane retardation (Rin), and a negative A-type panel for compensating out-of-plane retardation (Rout) which should be placed in a specific order to increase the contrast at wide viewing angles.
However, typical retardation films have a normal dispersion and cannot provide the solution to the above referenced disadvantages in the entire visible spectral range. It can result in the distortion of color of the displayed picture, especially at wide viewing angles. Usually the optimization of LCD is held in the maximal sensitivity human eye vision range for the light wavelength of 550 nm. Therefore, the maximal distortions arise in the red and blue parts of the light spectrum. In the present invention it is supposed that the visible spectral range has a lower boundary that is approximately equal to 400 nm, and an upper boundary that is approximately equal to 750 nm.
FIG. 5 (prior art) demonstrates a typical liquid crystal cell 1 of a color liquid crystal display. The liquid crystal cell comprises front substrate 2 with color filters 3, black matrix 4 and planarization layer 5; liquid crystal layer 6; other functional layers 7 comprising electrode and alignment layers; and a back substrate 8 with electrodes, driving elements and alignment layers.
The present invention provides a compensated color liquid crystal display with improved optical performance, in particular, higher contrast and better color rendering at a wide range of viewing angles, and reduced color shift in an entire viewing angle range. These advantages are provided along with the simplified manufacturing technology.